This application claims priority of German Application No. 199 46 176.7, filed Sep. 21, 1999 and International Application No. PCT/EP00/08826, filed Sep. 9, 2000, the complete disclosure of which is hereby incorporated by reference.
a) Field of the Invention
The invention is directed to a diode-pumped laser with internal frequency doubling in which a solid-state laser crystal with a temperature gradient directed substantially parallel to the cavity axis and a nonlinear optical crystal following in the direction of the beam path for converting the laser radiation with a fundamental laser wavelength into laser radiation of another wavelength are provided inside a laser cavity as active medium.
b) Description of the Related Art
A laser of the type mentioned above is known, for example, from S. Erhard et al., Trends in Optics and Photonics, Vol. 26, Advanced Solid-State Lasers, Martin M. Fejer, Hagop Injeyan, and Ursula Keller, eds. (Optical Society of America, Washington, D.C. 1999), pp. 38-44.
For various basic reasons, the conversion of laser radiation of a fundamental laser wavelength into laser radiation of a different wavelength, e.g., generation of the second harmonic by means of a nonlinear optical crystal, is burdened by problems of nonlinear dynamics resulting in an unstable output power behavior. The causes of this instability vary in nature.
The nonlinear coupling of laser modes through nonlinear optical processes in the frequency-doubling crystal leads to intensity fluctuations in the laser, or xe2x80x9cgreen problemxe2x80x9d, as it is called (T. Baer, J. Opt. Soc. Am. B3, 1175 (1986)).
The formation of thermal lenses in laser crystal and in frequency doubling crystal (KTP crystal) leads to a complex reaction of the output power as a function of the diode current, since the thermal lens influences the laser mode in the cavity and, therefore, the effectiveness of the frequency doubling which depends on the intensity of the laser field in the frequency doubling crystal. This prevents adjustment of power via the diode current.
The technical solutions offered by the prior art solve only partial problems without meeting the requirements as a whole.
For generating stable laser output powers, it is known to let laser cavities operate either in multimode ( greater than 100) operation or in monomode operation.
U.S. Pat. No. 5,446,749 provides a laser arrangement with a particularly long cavity. Operation with stable amplitude is achieved by means of exciting many longitudinal modes. The output power of the laser is not adjustable via the diode current because of the strong thermal lens formed in the utilized laser crystal rod, since a power fluctuation occurs in the generated second harmonic at the start of power adjustment. The particularly long cavity results in large dimensions of the laser produced in this way and entails high costs.
For monomode operation, it is known [S. Erhard et al., Trends in Optics and Photonics, vol. 26, Advanced Solid-State Lasers, Martin M. Fejer, Hagop Injeyan, and Ursula Keller, eds. (Optical Society of America, Washington, D.C. 1999), pp. 38-44] to generate the second harmonic through internal frequency doubling in a cavity of a Yb:YAG disk laser, as it is called, in which the utilized laser-active solid state medium is shaped like a thin disk whose dimension in the propagation direction of the laser radiation (laser axis) is sharply reduced in relation to the other dimensions. According to DE 43 44 227 A1, a laser crystal of this kind is fastened to a sturdy cooling element by its surface which is directed at right angles to the laser propagation direction. Accordingly, a temperature gradient predominantly parallel to the laser axis is formed in the crystal, so that the formation of an interfering thermal lens is sharply reduced. The described laser uses a long cavity (approximately 1 m) with a noncritical temperature phase-matched LBO crystal as nonlinear optical crystal. Monomode operation is compelled in a known manner in that etalons and birefringent filters are placed in the cavity to reduce the number of longitudinal modes. These are expensive, require very sensitive adjustment and cause intracavity losses, so that the effectiveness of the laser is reduced to the indicated 15.5%.
This object is met by a diode-pumped laser with internal frequency doubling in which a solid-state laser crystal with a temperature gradient directed substantially parallel to the cavity axis and a non-linear optical crystal following in the direction of the beam path for converting the laser radiation with a fundamental laser wavelength into laser radiation of another wavelength are provided inside a laser cavity as active medium, wherein the conversion of the laser radiation with fundamental laser wavelength into laser radiation of another wavelength is carried out with a lower non-linear conversion than required for achieving maximum power of the converted laser radiation.
U.S. Pat. No. 5,511,085 discloses another solution in the form of a microchip laser with small cavity length and intracavity nonlinear crystal. While the number of transverse modes is already limited by end-pumping, intracavity etalon effects further reduce the longitudinal mode spectrum through additional coatings on the laser crystal or nonlinear crystal. Strong thermal effects put increased stress on the optical elements, limit the laser output power and accordingly restrict the uses of the laser. This also makes adjustment of the arrangement more difficult.
It is the primary object of the invention to prevent interfering power fluctuations in the laser radiation generated by means of the nonlinear crystal at the start of the power adjustment in a simple and, therefore, inexpensive construction without power-reducing elements in the cavity or thermal effects having a negative impact on the laser behavior.
This object is met by a diode-pumped laser with internal frequency doubling in which a solid-state laser crystal with a temperature gradient directed substantially parallel to the cavity axis and a nonlinear optical crystal following in the direction of the beam path for converting the laser radiation with a fundamental laser wavelength into laser radiation of another wavelength are provided inside a laser cavity as active medium, wherein the conversion of the laser radiation with fundamental laser wavelength into laser radiation of another wavelength is carried out with a lower effectiveness than required for achieving maximum power of the converted laser radiation.
The effectiveness of the conversion can be adjusted substantially through the longitudinal dimensions of the nonlinear optical crustal along the cavity axis and should be in a range of 50% to 90% of the effectiveness at which the maximum power of the converted laser radiation can be achieved.
In contrast to the known solutions, a compact, short cavity is used in which a few modes are generated and in which additional mode-selective elements are dispensed with. As is well known, a step of this kind would not lead to a stable output power with intracavity frequency doubling. This stability is first achieved by the low conversion effectiveness by means of the nonlinear crystal which is constructed in a particularly way.
As a result of the construction of the laser-active medium as a disk-shaped solid-state laser crystal with low amplification and a substantial temperature gradient in the direction of the cavity axis, the disruptive effect of a thermal lens on the mode distribution is already sharply reduced. A residual action of a thermal lens forming in the laser-active medium in spite of the temperature gradient directed predominantly parallel to the cavity axis is eliminated in that its focal length is adjusted so as to be greater than the cavity length.
The principle of the disk laser is not used in the present invention to achieve the highest beam qualities for high-power lasers in continuous wave mode. The invention makes use of the characteristic of the stability of the thermal lens during changes in pump output which is required for the large dynamic range, i.e., a stable operation at very small outputs of about 20 mW, but also at very high outputs of about 4 W, as well as for switching ability (switch-on process).
The beam quality of the laser radiation generated by means of the nonlinear optical crystal is characterized by a diffraction number M2 between one and ten.
Various types of crystal can be used as nonlinear optical crystals. It is advantageous to use an LBO crystal with a crystal length of 2 mm to 10 mm through which a frequency doubling with critical angle phase matching is generated, which has a positive effect on the low conversion effectiveness to be used.
Various solid-state laser crystal such as Nd:YVO4 with an Nd doping of 0.5% to 2% and Nd:YAG with an Nd doping of 0.5% to 1.5% are suitable for the laser-active medium.
By reducing the crystal length, the effect occurring in the KTP crystal is the same as that occurring in the disk-shaped laser crystal, whereby the effect of a thermal lens is diminished. Further, this has the advantage of low adjustment sensitivity resulting in economical assembly. The temperature sensitivity and the minimizing of absorption losses are further characteristics of short frequency doubling crystals leading to advantages with respect to cost.
The laser cavity is advantageously folded by means of a folding mirror serving as out-coupling mirror and has a cavity axis directed to the folding mirror at an incident angle of less than 10xc2x0 to prevent astigmatism.
A recommended pump arrangement has at least one laser diode line which is either coupled with fiber optics or its pump radiation is transmitted by free-radiating optics.
It is also advantageous for forming a cavity end mirror when an end face of the optically nonlinear crystal directed away from the cavity interior is provided with a dielectric coating which is highly reflective for the laser fundamental wave and for the second harmonic generated by the nonlinear optical crystal.
In order to reduce losses, the cavity axis should be directed at right angles to crystal faces of the laser-active medium and of the nonlinear optical crystal, which faces work parallel to one another.
Various solid bodies such as Nd:YVO4 with an Nd doping of 0.5% to 2% and Nd:YAG with an Nd doping of 0.5% to 1.5% are suitable for the laser-active medium.
Nd:YALO crystals, Nd:YLF crystals or Nd:LSB crystals can also be used.
A more compact and economical construction can be realized by the invention, wherein output powers with stable amplitude and maximum fluctuations of 5% can be achieved. A laser output power in a range of 10 mW to 4 W and a square-pulse duration between 10 ms and continuous wave can be adjusted via the diode current, i.e., a preselected output power can be achieved in a stable manner in approximately 1 ms. A high overall effectiveness is achieved by means of the small quantity of intracavity elements which lead to losses. Due to the fact that strong thermal effects are prevented, the solution according to the invention is also suitable for high laser output powers such as are required, e.g., for applications in dermatology.